1. Field of the Invention
The present invention relates to an input/output device having a function to detect a type of a loaded interface board or the like.
2. Related Background Art
Conventional input/output devices having a function to detect an option board (expansion interface board) are classified into the following two types.
(1) a 3-bit signal line is used as an interface board detection signal line and a type of the option board is detected by "0" and "1" bits of the signal line. For example, "000" represents a serial interface, "001" represents a parallel interface, and "111" represents that no option board is loaded in the device.
(2) a 1-bit signal line for indicating the presence or absence of a card is provided as an interface board detection signal line. When the bit is "0", it represents that the option board is loaded in the device, and when the bit is "1", it represents that the option board is not loaded. If the option board is in the device, information indicating a type of card is detected from the data bus.
In each of the above devices, however, at least one card detection signal line is required, because if data is input from the option board while the card is not loaded, there is no response from the option board and a controller board waits for the response and is hung up. In order to prevent such a hang-up, it is necessary to detect the presence or absence of the card.
Besides the input/output devices described above, the presence or absence of the board may be detected by limiting the response time from the option board.
However, in order to comply with interfaces having various access time, the response time from the option board must be set to be long. Such a long response time is needed, for example, where a physical factor requires a long time at an initial access to a floppy disk or hard disk. In another exmaple, an object to be accessed has to wait during the input/output operation with other host computer.
By the above reasons, the design of the option board is difficult.